For machining a die or mold, there is often used a ball endmill having ball-nosed end cutting edges in its axially distal end portion. Conventionally, in such a ball endmill, each of the ball-nosed end cutting edges has an arcuate shape that is convex in a direction of rotation of the ball endmill for the purpose of improving its cutting performance. It is common that a radius of curvature of each arcuate-shaped ball-nosed end cutting edge is adapted to be constant from its inner peripheral portion to its outer peripheral portion (for example, see Patent Document 1). Patent Document 1: JP-H04-51928Y2